1. Field of the Invention
The present invention relates to a voice coil linear actuator, an apparatus using the actuator, and a method for manufacturing the actuator.
2. Description of the Related Art
Voice coil linear actuators are used in arrangements wherein linear actions are performed for relatively short distances, such as with a mounting head of an electronic parts mounting apparatus, for example.
Generally, voice coil linear actuators generate thrust by mutual action between a magnetic field from a magnet and an electric current flowing through a coil, and there are two general arrangements for this. One is an arrangement wherein the magnet is fixed and the coil moves, and the other is an arrangement wherein the coil is fixed and the magnet moves.
Arrangements disclosed in Japanese Unexamined Patent Application Publication No. 11-299210 and Japanese Unexamined Patent Application Publication No. 11-69755, for example, are known arrangements of the coil moving type. This conventional arrangement has a magnet which is magnetized in the radial direction and disposed on a surface of a yoke. The coil is disposed in the magnetic gap, and a current is applied to the coil, thereby generating thrust. Also, another embodiment disclosed in Japanese Unexamined Patent Application Publication No. 11-69755 describes an arrangement wherein a magnet which is magnetized in the axial direction is arranged in the axial direction of the center yoke, and yet another embodiment disclosed in Japanese Unexamined Patent Application Publication No. 11-69755 describes an arrangement having two magnets with opposite polarity which are magnetized in the radial direction and disposed on a surface of a yoke.
Now, it is desirable that actuators be capable of speedy acceleration and deceleration. Thrust must be increased to achieve this. Increasing the magnetic flux density of the magnetic gap or increasing the electric current flowing through the coil can be conceived as simple means for increasing the thrust. However, applying a great current to the coil increases the amount of heat generated at the coil, causing problems such as thermal deformation in the surrounding members, deterioration of the magnet due to the heat, and so forth. Accordingly, it is preferable to increase the magnetic flux density of the magnetic gap. Further, in the event of using the actuator in a mounting head for a parts mounting apparatus, reduction in the size and weight of the mounting head is required. That is to say, there is a demand for a voice coil actuator which has increased magnetic flux density to improve thrust, while being reduced in size. However, attempts to reduce the size of conventional voice coil linear actuators have had the following problems.
With conventional arrangements, there is only one magnetic circuit passing through the yoke, magnet, and armature coil. Accordingly, the maximum magnetic flux transmission is restricted by the path area of the magnetic flux in the yoke, so the magnetic flux density becomes saturated within the yoke, and the operating point of the magnet drops. Accordingly, obtaining high magnetic flux density is difficult, and great thrust cannot be obtained. Particularly, attempting to meet the demand for reducing the size of the actuator in the radial direction makes the center yoke narrower, and the outer yoke portion thin as well. Accordingly, obtaining high magnetic flux density becomes even more difficult.
Now, of the conventional structures shown in Japanese Unexamined Patent Application Publication No. 11-69755, the reason that a pair of magnets with opposite polarity are used is that the magnetic flux density increases due to the reduced magnetic resistance, thereby improving the thrust somewhat in comparison with the other conventional art. However, the effects thereof are limited, and there has also been the problem that this arrangement is difficult to assemble.